The present invention relates to a method and circuit for writing data in a static semiconductor memory device.
In bipolar type static RAM's (random access memory), memory cells are arranged at intersections of word lines and pairs of bit lines in a matrix. Each memory cell is formed of a flip-flop circuit including two transistors, wherein if one transistor is in the on state, the other is in the off state. When a certain word line is selected, the selected word line is maintained at a high level and the information stored in the memory cell is read out via the bit lines. That is, the bit lines are connected to readout transistors, the emitters of which are commonly connected to a constant current source, the bases of which are respectively connected to the bit lines, and the on and off state of the readout transistors is sensed by a sense amplifier.
Writing is carried out by turning on the transistor of the memory cell which is in the off state or by further turning off the transistor of the memory cell which is in the on state.
For the purpose of rapid writing in these bipolar type static RAM's one bit line is placed at a low potential level and the other bit line is placed at a high potential level. When this method is used, writing can be carried out by using write pulses having short pulse widths, whereby the writing operation can be accelerated.
In this method, the waveforms of potentials in various parts of the memory cell are accompanied by vibration, ringing or unwanted oscillation. When potentials applied to the write bit line amplifier are returned to the readout level after the completion of writing, they do not immediately change to the readout level, but first undergo damping vibrations for a while and then settle down. As the readout mode is produced in the memory on completion of writing and as any such vibrations will create a whisker-like peak in the output of the sense amplifier, there is a risk of error caused by such a peak when reading is effected just after writing. Such a whisker-like peak is caused by vibrations generated when one potential at a bit line falls and the other potential at another bit line rises. Especially large vibrations are generated when the potential falls.